The answer is dependent on the equipment you have available and the service provider in question.

There are a number of "Free" VoIP providers such as Free World Dialup, IPTel, SIPphone, and others. By signing up for one of these providers and purchasing an adapter or using free software on your PC, you can make calls to other subscribers of the same service free of charge.

You may also be able to make calls to a connected provider free of charge (e.g. Free World Dialup provides connectivity to SIPphone and others). Your only cost is the cost of the adapter, if you choose to buy one.

Other VoIP providers provide connectivity to the public switched telephone network. Depending on the provider, you may be able to make and/or receive calls from the PSTN.

You may have to pay a per-minute charge or get unlimited minutes within a particular calling area for a flat monthly fee. Calls that do not leave the VoIP providers network (e.g. to other subscribers or to voicemail) typically are free of charge.

You usually get a wide range of PSTN-like features included like Call Waiting, Call Forwarding, and Voicemail.

Plans can be anywhere from $5 up to $50 USD depending on the provider and the type of plan you're interested in, and typically includes the use of an adapter that allows you to hook up a normal telephone.